This invention relates to a resistance element for wound-type variable resistors and the method of making the same.
In general, the wound-type variable resistor uses as the resistance element a metallic resistance wire wound on a core, over which a slider is moved to obtain a desired resistance value. One of the disadvantages of such wound-type variable resistors is that sliding noise is caused as the slider slides over the resistance element while making contact with the element. "Sliding noise" is electrical noise, i.e., contact noise between the slider and the resistance element when the slider is moved relative to the resistance element. Contact Resistance Variation, CRV which is the maximum, instantaneous change in contact noise is a very common specification for an evaluation of electrical noise characteristic in wound-type variable resistors. Therefore, the work "sliding noise" is used to mean Contact Resistance Variation hereinafter. Some of the wound-type variable resistors of the conventional type generate sliding noise immediately after being assembled. Other types of variable resistors produce sliding noise after a lapse of time or after exposure to elevated temperatures. In such variable resistors, sliding noise may be temporarily suppressed by reciprocating the slider over the resistance element several times, but the once-suppressed noise tends to be generated again after a lapse of time or after exposure to high-temperature atmosphere. Such sliding noise is attributable to the uneven contact resistance between the slider and the resistance element resulting from the formation of a film on the surface of the resistance wire by metallic oxides, chlorides and sulfides, together with various organic molecules, adsorbed gases and other contamination, which act as insulators on the surface of the resistance element. Particularly, the formation of a surface film by metallic oxides, chlorides and sulfides is attributable partly to oxidation by the outside atmosphere, or to the formation of intermetallic compounds by the ingredients (for example, iron sulfides, iron chlorides, etc.) of lubricant used in the die-drawing of metallic resistance wires.
As methods for preventing the sliding noise resulting from the formation of a film on the surface of the resistance wire, technologies disclosed in the Japanese Utility Model Publication No. 27447 of 1963 and the Japanese Utility Model Publication No. 33081 of 1979 have been proposed. The former is concerned with the method in which a hard-to-oxidize electrically conductive layer is formed by applying a silver-plating film or silver paste to the portion at which the resistance wire makes contact with the slider to prevent the oxidation of that portion. The latter, on the other hand, involves the method in which a resistance wire coated with an electrically conductive resin film is wound on a core to prevent the oxidation of the portion at which the resistance wire makes contact with the slider. Because of the increased cost of the resistance wire involved, both methods are considered impracticable, except for use in special-purpose variable resistors. Furthermore, these conventional technologies are concerned solely with prevention of the oxidation of the resistance wire by the outside atmosphere, and not with preventing the sliding resistance which is increased by the formation of alloys on the resistance wire surface by the ingredients of lubricant during the die-drawing of the resistance wire.
In the specification of the U.S. Pat. No. 3,697,920, a technology of depositing aluminum silicate on the surface of the resistance element to reduce sliding noise between the resistance element and the slider has been disclosed. Although the reason why sliding noise can be reduced by disposing aluminum silicate on the resistance element surface has not been made clear in the U.S. patent specification, it is evident that the patent is not intended to remove a film on the surface of the resistance wire formed by the ingredients of lubricant during the drawing of the resistance element.
There can be another method of reducing sliding noise in wound-type variable resistors by increasing the contact pressure of the slider with the resistance element. In a resistance element (for use in a small-sized wound-type variable resistor having a total resistance value of 20 kohms, for example) in which a resistance wire as fine as, say, approx. 16 microns in diameter is used, it is impossible to increase the contact pressure because of possible breakage of the resistance wire.